The present invention relates to an exhaust pipe, and to a method of making an exhaust pipe.
Internal combustion engines are generally equipped heretofore with a three-way catalytic converter for emission control. Three-way catalytic converters require a particular starting temperature to assume the actual emission control function. Regulatory exhaust standards demand an increasingly shorter time period for the catalytic converter to reach the starting temperature, the so-called “light of time”, whereby an operating temperature of >350° C. should be reached in shortest possible time.
The use of air gap insulated exhaust pipes between the cylinder outlet and the catalytic converter is one approach to conform to these standards. Air gap insulated exhaust pipes are typically designed in double-walled configuration and include a thin-walled inner pipe portion and an outer pipe portion of greater wall thickness, which are separated from one another by an air gap. The insulating effect of the air gap, the small mass of the inner pipe and the resultant low heat capacity of the inner pipe cause minimal heat loss so that the catalytic converter can quickly reach in the starting phase the required starting temperature. The outer pipe provides hereby a supporting and sealing function.
Air gap insulated exhaust pipes have also been used to connect the cylinder discharge with an exhaust turbocharger. The reduced heat loss of an air gap insulated exhaust pipe results in a withdrawal of less energy from the exhaust during its paths to the exhaust turbocharger compared to configurations without air gap insulation, so that more energy is available at the exhaust turbocharger for conversion. An air gap insulated exhaust pipe thus enhances the efficiency of an exhaust turbocharger.
The attachment of the exhaust pipe between two fixed attachment points, e.g., an exhaust manifold and an exhaust turbocharger, poses, however, a problem. The outer pipe should be able to compensate its own thermal expansion as well as possible thermal expansions of adjacent components, such as the manifold or the exhaust turbocharger. In addition, the rapid temperature increase in an exhaust pipe results in a temperature drop between the inner pipe and the outer pipe so that additional relative movements between inner and outer pipes have to be compensated. The use of expansion compensating members has been proposed to address this demand in order to allow a relative movement between the inner and outer pipes.
It would be desirable and advantageous to provide an improved exhaust pipe which obviates prior art shortcomings and which is able to compensate in a simple manner thermal length changes.
It would also be desirable and advantageous to provide an improved method for manufacture of an exhaust pipe, which is gentle to the material used and allows easy assembly.